An acoustic wave device is used as a filter, a duplexer or the like in mobile communication terminals. Acoustic wave resonators include a surface acoustic wave (SAW) resonator using a surface acoustic wave, a boundary acoustic wave resonator using a boundary acoustic wave, a piezoelectric thin-film resonator using a piezoelectric thin-film. A film bulk acoustic wave resonator (FBAR) is an exemplary piezoelectric thin-film resonator. A functional portion for excitation of acoustic waves includes an electrode such as an interdigital transducer (IDT) in the SAW device, and is a portion in which electrodes face each other through a piezoelectric thin-film. A substrate on which an acoustic wave element is formed may be sealed with resin or the like for the purpose of downsizing the acoustic wave device and protecting the acoustic wave element.
Japanese Patent Application Publication No. 2008-227748 (Document 1) discloses sealing with epoxy resin. The material for sealing may be polyimide, for example. However, polyimide has a high hygroscopic property and may not protect the acoustic wave element sufficiently. Japanese Patent Application Publication No. 9-326447 (Document 2), Japanese Patent Application Publication No. 9-174786 (Document 3) and Japanese Patent Application Publication No. 2008-103559 (Document 4) disclose sealing with liquid crystal polymer. Liquid crystal polymer has a low hygroscopic property, as compared with polyimide resin, and is thus suitable for the sealing substance. As described in paragraph 0002 of Document 3, in a case where the sealing substance is formed into a sheet, the liquid crystal polymer is oriented in the sheet extruding direction in which a sheet is extruded by rollers of a machine. The liquid crystal sheet has physical properties that greatly depend on the orientation direction. Generally, the hygroscopic property and the adhesive property are those obtained under the condition that the orientation direction corresponds to the sheet extruding direction.
However, the combination of liquid crystal polymer and the base member to which liquid crystal polymer is bonded or the conditions in adhesion may not have the desired adhesive property. If the adhesive property is insufficient, moisture may enter through the interface between the liquid crystal polymer and the base member, and the electronic components may not be protected sufficiently (see paragraph 0002 of Document 4).